This invention relates generally to compressor stages, and more specifically to a compressor stage having inducer vents.
Compressor stages, such as those used for engine turbocharging, experience surge and choking at various mass flow rates. Surge generally occurs as the compressor stage starts to experience violent instability or flow reversals. Conversely, choking generally occurs as the maximum mass flow rate through the compressor stage passes at a certain compressor speed. This maximum flow is considered below the sixty-eight percent efficiency in the present day technology. The surge line and the choke line are typically plotted on a pressure ratio-mass flow rate graph known as a compressor map. A compressor map illustrates the range in which a given compressor design may operate without surge or choking.
The parameters that determine the onset or the start of a stall or a surge in a compressor stage are the blade shape of the impeller, the inducer diameter of the impeller, the tip width of the discharge or trailing edge of the impeller, the diffuser width, the shape and size of the volute, and the surface roughness of the surfaces of the diffuser, volute, and associated passages. Reverse flow initiated because of stall traverses back all the way to the inducer and even farther away from the inducer.
It is desirable to enlarge the range of operation (or map width) of a compressor mainly before surge occurs to improve compressor performance, particularly for certain demanding applications. Performance improvements may include expanding the range of conditions and speeds in which the compressor will operate, increasing efficiency, increasing available power, and decreasing the air noise regime associated with surge phenomenon.
One approach that has been used to address this problem has been to provide a bidirectional bleed hole or vent located in the inducer shroud of the compressor. This bleed slot (or series of circumferentially aligned holes, or single circumferential slot) acts to allow an inflow of air during what would otherwise be choke conditions, and acts to allow an outflow of air during what would otherwise be surge conditions. The result, when plotted on a compressor map, is to shift the surge line to the left on the compressor map and/or to shift the choke line to the right on the compressor map. Thus, the range of operation between compressor stage surge and choke is enlarged.
The present invention significantly enlarges the range of operation between compressor stage surge and choke beyond prior techniques. The present invention especially improves the surge characteristics of a compressor. Furthermore, the present invention affords compressor designers greater latitude in dictating the surge line profile and/or choke line profile for a given compressor design. Thus, compressor designers may, for a particular application, better tailor compressor surge characteristics to suit the particular application. Accordingly, the present invention is a significant advance in the compressor art.
The present invention accomplishes these advantages by including multiple location bleed holes along the inducer and contour of the compressor stage. By selectively locating these bleed holes with respect to the leading edge of the impeller blades, and with respect to each other, and by selectively determining the width of these holes, significant surge line movement can be obtained. By selectively determining the size of these bleed holes (slots in the preferred embodiment), surge lines can be contoured to ones needs.